The invention relates to cross-linkable, olefinic polymers from olefinic polymers and ethylenically unsaturated organosilane compounds, as well as to methods for their synthesis.
For various applications, the properties of olefinic polymers, such as polyethylene (PE), polypropylene (PP), 1-polybutene (PB), poly-4-methylpentene (PMP), polystyrene (PS) and of the corresponding copolymers are inadequate. In particular, the strength, toughness, dimensional stability under heat and the resistance to various chemicals of these polymers do not satisfy many requirements in the field of producing structural parts.
By introducing covalent bridging bonds between the polymer chains by way of chain branchings and cross linkings, the property spectrum of these polymers can be extended. For example, the cross linking of PE causes the toughness and dimensional stability under heat to be increased over that of the original material and the resistance to chemicals to be improved. For some applications, partial cross linking of the polymer is also sufficient or advantageous, for example, when a higher melt viscosity is required.
On the one hand, the formation of covalent bonds between chains of olefinic polymers is achieved according to the state of the art by free radical linking reactions. The linking can take place here by a combination of two chain radicals, which have come about by reaction with free radicals of decomposed peroxides or azo compounds or by the action of high-energy radiation and/or by the reaction of the chain radicals with di-unsaturated or polyunsaturated organic compounds, such as divinylbenzene, phenyl bis(maleic imide) or pentaerythritol tetraethyl ether. The latter are known as common cross-linking reagents.
A further variation of covalent linkage are condensations between functional groups of the polymers, which either are incorporated as comonomers in the polymer chain or bound to the polymer chain by grafting reactions. For example, PE can be functionalized by free radical grafting with vinyltrialkoxysilanes. By a subsequent, catalyzed hydrolysis and a catalyzed condensation, the PE is cross linked over siloxane bridges (Plastics and Rubber Processing and Applications 13 (1990) 81-91).
According to the state of the art, the grafting of olefinic polymers with ethylenically unsaturated organosilane compounds takes place with addition of thermally decomposing free radical-forming agents or under the action of ionizing radiation. The reaction can take place in inert solvents, in the melt or in the particulate, solid polymer. Grafting in solution is very expensive and has not gained any economic importance.
The grafting of organosilane compounds on polyolefins in the melt for the synthesis of polyolefins, cross-linkable by way of grafted organosilane compounds, is used industrially in the presence of peroxides as thermally decomposing free radical-forming agents only for polyethylene and ethylene copolymers. However, because of various side reactions, there are disadvantages. For example, during the free radical silane grafting of polyethylene in the melt, the formation of cross-linked portions by a combination of free PE chain radicals takes place. This may interfere with processing because of a lower melt index. A direct transfer of this method to polypropylene fails owing to the fact that, because of the tertiary carbon atoms in the molecular structure of the polypropylene at higher temperatures, P chain splitting takes place (Fritz, H., Polymerwerkstoffe Merseburg 1994, 23-32) and the peroxide initiated decomposition of the polypropylene chain predominates over the peroxide grafting of the organosilane compound (Ambrosh, I., "Polypropylen", published by Khimiya Moscow, 1967, page 125). The discoloration of the polymers, which frequently occurs during the melt grafting reaction, is also a disadvantage.
According to the German patent 2617108, in the first step of melt grafting, the ethylenically unsaturated organosilane compound, the free radical forming agent and other additives diffuse under the intensive mixing of the components at 60.degree. to 100.degree. C. The grafting of the ethylenically unsaturated organosilane compound, initiated by the free radical-forming agent, takes place in the second step of melt grafting in the extruder at 180.degree. to 220.degree. C.
The diffusion of the ethylenically unsaturated organosilane compound and of the peroxide are not always carried out as separate steps of the method.
According to J5434799, for a single step extrusion method, the ethylenically unsaturated organosilane compound and the peroxide are metered simultaneously with the olefinic polymer into the extruder and the free radical grafting is initiated with melting of olefinic polymer, mixing of the components and a rise in the temperature.
As a result of the inadequate homogenization of the olefinic polymer, the ethylenically unsaturated organosilane compound and the free radical-forming agent, this technology of the German patent 2617108 and of J5434799 has the disadvantage that cross linking takes place in local regions and results in difficulties during the processing.
Furthermore, for the grafting of organosilane compounds initiated by peroxides, a thermooxidative degradation of the olefinic polymers cannot be excluded at reaction temperatures above 200.degree. C., while the addition of stabilizers brings out about a decrease in the degree of grafting of the organosilane compound (Voight, I., "Stabilization of Synthetic Polymers Against the Effects of Light and Heat", published by Khimiya Moscow 1972, P.59).
The highly toxic splitting products, which are formed by a series of peroxides and which lead to a decrease in the dielectric properties of the final products, are an additional problem.
The grafting of ethylenically unsaturated monomers, including ethylenically unsaturated organosilane compounds, to olefinic polymers in the solid phase in the presence of thermally decomposing free radical-forming agents or under the action of ionizing radiation (gamma radiation, X-radiation, accelerated electrons), is also known.
For the grafting of olefinic polymers in the presence of thermally decomposing free radical-forming agents in the solid phase, particulate olefinic polymers with ethylenically unsaturated monomers and thermally decomposing free radical-forming agents are mixed together in one or several steps, heated to a specified reaction temperature and reacted to form grafted copolymers (DE 41 23 972, DD 135 622, DD 131 752, DD 135 621).
The above-mentioned undesirable side reactions can also occur when grafting is carried out in the solid phase. In the U.S. Pat. No. 4,595,726, reference is made to the simultaneously occurring degradation of polypropylene, which becomes noticeable due to the increase in the melt index. Homopolymerization of the ethylenically unsaturated monomers used is another side phenomenon. Homopolymers can also be formed at the surface of the polymer particles that are to be grafted and cause agglomeration of the solid particles, as a result of which the flowability is adversely affected. As a consequence, the polymer bakes to the reactor wall and forms undesirable agglomerates. A further disadvantage consists therein that the grafting of the ethylenically unsaturated monomers does not take place uniformly over the cross section of the particles.
For the grafting of olefinic polymers under the action of ionizing radiation in the solid phase, vinyl monomers, such as styrene, butyl methacrylate, butyl acrylate and benzyl methacrylate were grafted according to EP 437 808 on polyolefins, including polypropylene, according to the "pre-irradiation method", polyolefin powder or polyolefin granulates being subjected at 10.degree. to 85.degree. C. to a pre-irradiation under inert conditions at radiation doses of 1 to 12 Mrd. Subsequently, grafting was accomplished by contacting with the liquid monomer or with the monomer solution at 10.degree. to 50.degree. C.
For the pre-irradiation method, polymer radicals are formed in the total volume of the polymer particles. These polymer radicals form the starting point for the subsequent grafting of the ethylenically unsaturated monomers. The method is used to improve the adhesion properties and dyeability of olefinic polymers and to achieve ion-exchange properties in these polymers.
According to EP 437 808, polymer degradation is prevented by excluding oxygen during the irradiation of olefinic polymers. However, only very reactive ethylenically unsaturated monomers such as styrene or acrylates, can be grafted in good yield by the pre-irradiation method.
Less reactive ethylenically unsaturated monomers, such as vinylsilanes, cannot be grafted by the pre-irradiation method.
DT 135 499 describes the grafting of olefinic polymers, including polypropylene in a fluidized bed by the pre-irradiation method and by the simultaneous irradiation method (pre-absorption). In the case of polypropylene, there was only surface grafting when vinylsilanes were used, the degree of grafting being too low to achieve wettability of the polypropylene by moisture.
DE 24 39 514 discloses the production of cross-linkable polyethylene high-voltage cables by the pre-absorption of vinylsilane in polyethylene in a special mixer and the continuous irradiation of the cable at room temperature. Under these conditions, the degradation of the polyethylene by irradiation was low.
It is an object of the present invention to develop cross-linkable olefinic polymers, which avoid the described disadvantages of known cross-linkable olefinic polymers, such as inhomogeneities and undesirable by-products in the modified product, which generally become noticeable during use, as well as to develop methods for their synthesis.